The present inventors are building a spectrophotometer, an instrument that measures the transmission or reflectivity of a sample as a function of wavelength. In general, the components of such a system are a light source with a spectral range sufficient to cover the wavelength band of interest, a sample compartment and associated optics to capture the light from the source, transmit it through or reflect it off of the sample and then concentrate the transmitted or reflected light on the input slit of a monochromator or other device to select a particular wavelength, and finally a detector sensitive to all wavelengths of interest. Most commercially available spectrophotometers are built as an integrated unit containing all of the above components. This makes it possible to optimize the instrument for making certain measurements but at the same time limits the flexibility of the instrument.
The detector and monochromator (or spectral filtering device) are common to all spectrophotometers. On the other hand, if there is only one path from the source to the monochromator, it is difficult to calibrate the spectrophotometer as a function of wavelength to account for changes in the transmission and reflectivity of the optics with wavelength. Even more important, in the infrared, it is difficult to account for the rather sharp absorption bands in the atmosphere that severely affect the light reaching the detector. The device described below is intended for use in the infrared from about 2 to 50 μm. On the other hand, it could be extended to other wavelengths.
For this reason it is useful to temporally divide the light from the source to first go down one path to the detector and then a second essentially identical path to the detector. In this way to measure the transmission of a sample, for example, the light can pass through the sample in one path and through nothing but air in the other. Ultimately, the detector sees an alternating signal of the light through the sample versus no sample, or 100% transmission. Now as the wavelength is scanned through an atmospheric absorption band the effect of this band is imposed on both paths identically so that only the effect of the sample is seen in the measurement.
To provide for the two light paths and the recombination of these paths prior to the light entering the monochromator, there are shutters or choppers before and after the sample, the first to divide and direct the light first down one path, then the other. A reflecting chopper is used at the recombining end to let one beam fall on the monochromator input slit where there is a hole in the chopper blade and to reflect the second beam onto the slit when the blade is a plane mirror surface.
At this point, matters are fairly simple but when it is desired to measure both transmission and reflectivity with the same instrument, things become complicated. More complex yet are measurements of diffuse transmission and reflection where integrating spheres must be inserted in the optical path. Thus, there is a need for a mechanism to deal with these complexities by allowing the dual beam sample system to be assembled in the user's lab or production facility as needed for any particular measurement task, including robotically loading and unloading samples of various geometries and having the modules properly aligned both optically and mechanically.